Golf ball

ABSTRACT

A golf ball  2  includes a core  4,  an inner cover  6,  and an outer cover  8.  The golf ball  2  satisfies the following mathematical formulas (1), (2), and (3): 
       5≤( Hi*Ti−Dc*Rc )≤40  (1);
 
       5≤( Hi*Ti−Ho*To )≤40  (2); and
 
       −20≤( Ho*To−Dc*Rc )≤20  (3).
 
     Dc is the amount of compressive deformation (mm) of the core  4,    
     Rc is the radius (mm) of the core  4,    
     Hi is the Shore C hardness of the inner cover  6,    
     Ti is the thickness (mm) of the inner cover  6,    
     Ho is the Shore C hardness of the outer cover  8,  and 
     To is the thickness (mm) of the outer cover  8.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority on and the benefit of Patent Application No. 2021-106295 filed in JAPAN on Jun. 28, 2021. The entire disclosures of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present specification discloses a golf ball including a core, an inner cover, and an outer cover.

Description of the Related Art

A typical golf ball includes a core, an inner cover, and an outer cover. The core is formed by crosslinking a rubber composition. The inner cover is formed from a resin composition. The outer cover is formed from another resin composition.

The face of a golf club has a loft angle. When a golf ball is hit with the golf club, the golf ball is launched at a launch angle corresponding to the loft angle. Furthermore, in the golf ball, backspin due to the loft angle occurs. The golf ball flies with the backspin.

Various improvements have been proposed regarding the structures and materials of golf balls for the purpose of improving flight performance and other properties. An example of the improvements is disclosed in Japanese Laid-Open Patent Application Publication No. 2021-74198.

Average-skilled golf players generally exhibit low head speeds in driver shots. For such a player, a golf ball is suitable which is sufficiently deformable even when hit with a golf club swung at a low head speed. The golf ball hit by the player flies at a low speed. The flight distance of the golf ball hit with a low head speed does not greatly depend on the flight speed. The flight distance of the golf ball hit with a low head speed greatly depends on the spin rate. Golf balls that are not likely to spin are advantageous for average-skilled players in terms of flight distance in driver shots.

Golf players place importance also on the controllability of golf balls. The controllability is correlated with the spin performance. The higher the backspin rate of a golf ball is, the smaller the run of the golf ball is. With the use of a golf ball to which high-rate backspin can be applied, the player can stop the golf ball at a target point. The higher the sidespin rate of a golf ball is, the more likely the golf ball is to curve. With the use of a golf ball to which high-rate sidespin can be applied, the player can intentionally cause the golf ball to curve. Players place particular importance on the controllability in approach shots.

A golf ball having a soft cover is excellent in the controllability in approach shots. However, this cover impairs the resilience performance of the golf ball. In this golf ball, the use of a thin cover can reduce the adverse effect of the cover on the resilience performance. Furthermore, in this golf ball, a high-hardness core or intermediate layer can compensate for the adverse effect of the cover on the resilience performance. However, this golf ball gives hard feel at impact.

The present applicant aims to provide a golf ball excellent in flight performance and feel at impact in driver shots and excellent in controllability and feel at impact in approach shots.

SUMMARY OF THE INVENTION

A preferred golf ball includes: a core; an inner cover positioned outside the core; and an outer cover positioned outside the inner cover. The golf ball satisfies the following mathematical formulas (1), (2), and (3):

5≤(Hi*Ti−Dc*Rc)≤40  (1);

5≤(Hi*Ti−Ho*To)≤40  (2); and

−20≤(Ho*To−Dc*Rc)≤20  (3).

Dc is an amount of compressive deformation (mm) of the core,

Rc is a radius (mm) of the core,

Hi is a Shore C hardness of the inner cover,

Ti is a thickness (mm) of the inner cover,

Ho is a Shore C hardness of the outer cover, and

To is a thickness (mm) of the outer cover.

In driver shots of the golf ball, the hardness distribution of the sphere including the core and the inner cover contributes to a large launch angle and a low spin rate. In driver shots of the golf ball, the outer cover contributes to soft feel at impact. The golf ball is excellent in flight performance and feel at impact in driver shots.

In approach shots of the golf ball, the outer cover contributes to a high spin rate. In approach shots of the golf ball, the outer cover contributes to soft feel at impact. The golf ball is excellent in controllability and feel at impact in approach shots.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a partially cut-away cross-sectional view showing a golf ball according to one embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail preferred embodiments with appropriate reference to the FIGURE.

A golf ball 2 shown in the FIGURE includes a spherical core 4, an inner cover 6 positioned outside the core 4, and an outer cover 8 positioned outside the inner cover 6. The golf ball 2 has a plurality of dimples 10 on the surface thereof. Of the surface of the golf ball 2, a part other than the dimples 10 is a land 12. The golf ball 2 includes a paint layer and a mark layer on the external side of the outer cover 8. These layers are not shown in the FIGURE.

The golf ball 2 preferably has a diameter of not less than 40 mm and not greater than 45 mm. In light of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably not less than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably not greater than 44 mm and particularly preferably not greater than 42.80 mm.

The golf ball 2 preferably has a weight of not less than 40 g and not greater than 50 g. In light of attainment of great inertia, the weight is more preferably not less than 44 g and particularly preferably not less than 45.00 g. In light of conformity to the rules established by the USGA, the weight is particularly preferably not greater than 45.93 g.

The core 4 is formed by crosslinking a rubber composition. Examples of preferred base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of the resilience performance of the golf ball 2, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferable if the polybutadiene is a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably not less than 50% by weight, more preferably not less than 70% by weight, and particularly preferably not less than 80% by weight. A polybutadiene having a cis-1,4 bond content of not less than 80% is particularly preferred.

The rubber composition of the core 4 preferably includes a co-crosslinking agent. Co-crosslinking agents preferred in light of the durability and resilience performance of the golf ball 2 are monovalent or bivalent metal salts of α, β-unsaturated carboxylic acids having 2 to 8 carbon atoms. Examples of preferred co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. Zinc acrylate and zinc methacrylate are particularly preferred.

The rubber composition may include a metal oxide and an α, β-unsaturated carboxylic acid having 2 to 8 carbon atoms. They react with each other in the rubber composition to give a salt. The salt serves as a co-crosslinking agent. Examples of preferred α, β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of preferred metal oxides include zinc oxide and magnesium oxide.

The amount of the co-crosslinking agent per 100 parts by weight of the base rubber is preferably not less than 10 parts by weight and not greater than 45 parts by weight. The golf ball 2 in which this amount is not less than 10 parts by weight is excellent in resilience performance. From this viewpoint, this amount is more preferably not less than 15 parts by weight and particularly preferably not less than 20 parts by weight. The golf ball 2 in which this amount is not greater than 45 parts by weight is excellent in feel at impact. From this viewpoint, this amount is more preferably not greater than 40 parts by weight and particularly preferably not greater than 35 parts by weight.

Preferably, the rubber composition of the core 4 includes an organic peroxide. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the durability and resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. An organic peroxide with particularly high versatility is dicumyl peroxide.

The amount of the organic peroxide per 100 parts by weight of the base rubber is preferably not less than 0.1 parts by weight and not greater than 3.0 parts by weight. The golf ball 2 in which this amount is not less than 0.1 parts by weight is excellent in resilience performance. From this viewpoint, this amount is more preferably not less than 0.3 parts by weight and particularly preferably not less than 0.5 parts by weight. The golf ball 2 in which this amount is not greater than 3.0 parts by weight is excellent in feel at impact. From this viewpoint, this amount is more preferably not greater than 2.5 parts by weight and particularly preferably not greater than 2.0 parts by weight.

Preferably, the rubber composition of the core 4 includes an organic sulfur compound. The organic sulfur compound contributes to the resilience performance of the golf ball 2. Examples of the organic sulfur compound include naphthalenethiol compounds, benzenethiol compounds, and disulfide compounds.

Examples of the naphthalenethiol compounds include 1-naphthalenethiol, 2-naphthalenethiol, 4-chloro-1-naphthalenethiol, 4-bromo-1-naphthalenethiol, 1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol, 1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, 1-acetyl-2-naphthalenethiol, and metal salts of these compounds. Preferred metal salts are zinc salts.

Examples of the benzenethiol compounds include benzenethiol, 4-chlorobenzenethiol, 3-chlorobenzenethiol, 4-bromobenzenethiol, 3-bromobenzenethiol, 4-fluorobenzenethiol, 4-iodobenzenethiol, 2,5-dichlorobenzenethiol, 3,5-dichlorobenzenethiol, 2,6-dichlorobenzenethiol, 2,5-dibromobenzenethiol, 3,5-dibromobenzenethiol, 2-chloro-5-bromobenzenethiol, 2,4,6-trichlorobenzenethiol, 2,3,4,5,6-pentachlorobenzenethiol, 2,3,4,5,6-pentafluorobenzenethiol, 4-cyanobenzenethiol, 2-cyanobenzenethiol, 4-nitrobenzenethiol, 2-nitrobenzenethiol, and metal salts of these compounds. Preferred metal salts are zinc salts.

Examples of the disulfide compounds include diphenyl disulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(4-cyanophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl) disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, bis(2-cyano-5-bromophenyl)disulfide, bis(2,4,6-trichlorophenyl)disulfide, bis(2-cyano-4-chloro-6-bromophenyl)disulfide, bis(2,3,5,6-tetrachlorophenyl)disulfide, bis(2,3,4,5,6-pentachlorophenyl)disulfide, and bis(2,3,4,5,6-pentabromophenyl)disulfide.

The amount of the organic sulfur compound per 100 parts by weight of the base rubber is preferably not less than 0.1 parts by weight and not greater than 1.5 parts by weight. The golf ball 2 in which this amount is not less than 0.1 parts by weight is excellent in resilience performance. From this viewpoint, this amount is more preferably not less than 0.2 parts by weight and particularly preferably not less than 0.3 parts by weight. The golf ball 2 in which this amount is not greater than 1.5 parts by weight is excellent in feel at impact. From this viewpoint, this amount is more preferably not greater than 1.3 part by weight and particularly preferably not greater than 1.1 parts by weight. Two or more organic sulfur compounds may be used in combination.

Preferably, the rubber composition of the core 4 includes a carboxylic acid or a carboxylate. The carboxylic acid and the carboxylate can contribute to achieving an appropriate hardness distribution of the core 4. The appropriate hardness distribution can reduce the spin rate in driver shots. An example of preferred carboxylic acids is benzoic acid. Examples of preferred carboxylates include zinc octoate and zinc stearate. The amount of the carboxylic acid or carboxylate per 100 parts by weight of the base rubber is preferably not less than 0.5 parts by weight, more preferably not less than 0.8 parts by weight, and particularly preferably not less than 1.0 part by weight. This amount is preferably not greater than 20 parts by weight, more preferably not greater than 15 parts by weight, and particularly preferably not greater than 10 parts by weight.

The rubber composition of the core 4 may include a filler for purposes such as specific gravity adjustment. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 4 is achieved.

The rubber composition of the core 4 may include various additives such as sulfur, an anti-aging agent, a coloring agent, a plasticizer, and a dispersant in appropriate amounts. The rubber composition may include crosslinked rubber powder or synthetic resin powder.

The core 4 preferably has a diameter of not less than 35.0 mm and not greater than 40.5 mm. In other words, the radius Rc of the core 4 is preferably not less than 17.5 mm and not greater than 20.25 mm. The golf ball 2 that includes the core 4 having a diameter of not less than 35.0 mm is excellent in resilience performance. From this viewpoint, the diameter is more preferably not less than 36.0 mm and particularly preferably not less than 36.5 mm. The golf ball 2 that includes the core 4 having a diameter of not greater than 40.5 mm is excellent in durability. From this viewpoint, the diameter is more preferably not greater than 40.0 mm and particularly preferably not greater than 39.5 mm.

The core 4 preferably has an amount of compressive deformation Dc of not less than 3.1 mm and not greater than 5.6 mm. The golf ball 2 that includes the core 4 having an amount of compressive deformation Dc of not less than 3.1 mm is excellent in feel at impact. From this viewpoint, the amount of compressive deformation Dc is more preferably not less than 3.3 mm and particularly preferably not less than 3.5 mm. The golf ball 2 that includes the core 4 having an amount of compressive deformation Dc of not greater than 5.6 mm is excellent in resilience performance. From this viewpoint, the amount of compressive deformation Dc is more preferably not greater than 5.4 mm and particularly preferably not greater than 5.2 mm.

The amount of compressive deformation Dc is measured using a YAMADA type compression tester “SCH”. In the tester, the core 4 is placed on a hard plate made of metal. A cylinder made of metal gradually descends toward the core 4. The core 4 is squeezed between the bottom face of the cylinder and the hard plate and thus deformed. The distance is measured through which the cylinder moves from when an initial load of 98 N is applied to the core 4 to when a final load of 1274 N is applied to the core 4. The moving speed of the cylinder is 0.83 mm/s until the initial load is applied. The moving speed of the cylinder is 1.67 mm/s after the initial load is applied and until the final load is applied.

The inner cover 6 is positioned outside the core 4. In the present embodiment, the inner cover 6 is in contact with the core 4. The inner cover 6 is formed from a resin composition. In the present embodiment, the inner cover 6 is formed from a thermoplastic resin composition. Examples of the base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. Ionomer resins are particularly preferred. Ionomer resins are highly elastic. The golf ball 2 that includes the inner cover 6 containing an ionomer resin is excellent in resilience performance. The golf ball 2 is excellent in flight performance in driver shots.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is used as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably not less than 50% by weight.

Examples of preferred ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferred binary copolymer includes 80 to 90% by weight of an α-olefin and 10 to 20% by weight of an α,β-unsaturated carboxylic acid. The binary copolymer is excellent in resilience performance. Examples of other preferred ionomer resins include ternary copolymers formed with an α-olefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferred ternary copolymer includes 70 to 85% by weight of an α-olefin, 5 to 30% by weight of an α,β-unsaturated carboxylic acid, and 1 to 25% by weight of an α,β-unsaturated carboxylate ester. The ternary copolymer is excellent in resilience performance. For the binary and ternary copolymers, preferred α-olefins are ethylene and propylene, and preferred α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferred ionomer resin is a copolymer formed with ethylene and acrylic acid. Another particularly preferred ionomer resin is a copolymer formed with ethylene and methacrylic acid.

In the binary and ternary copolymers, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for neutralization include sodium ions, potassium ions, lithium ions, zinc ions, calcium ions, magnesium ions, aluminum ions, and neodymium ions. The neutralization may be effected with two or more types of metal ions. Metal ions particularly suitable in light of the resilience performance and durability of the golf ball 2 are sodium ions, zinc ions, lithium ions, and magnesium ions.

Specific examples of the ionomer resin include: trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1855”, “Himilan 1856”, “Himilan 8150”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7329”, and “Himilan AM7337”, manufactured by DOW-MITSUI POLYCHEMICALS CO., LTD.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF 1000”, and “HPF 2000”, manufactured by E.I. du Pont de Nemours and Company; and tread names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation. Two or more ionomer resins may be used in combination.

A preferred resin that can be used in combination with the ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer includes a polystyrene block as a hard segment and a soft segment. A typical soft segment is a diene block. Examples of compounds for the diene block include butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two or more compounds may be used in combination.

Examples of the styrene block-containing thermoplastic elastomer include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenated SBS, hydrogenated SIS, and hydrogenated SIBS. Examples of the hydrogenated SBS include styrene-ethylene-butylene-styrene block copolymers (SEBS). Examples of the hydrogenated SIS include styrene-ethylene-propylene-styrene block copolymers (SEPS). Examples of the hydrogenated SIBS include styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of the resilience performance of the golf ball 2, the content of the styrene component in the styrene block-containing thermoplastic elastomer is preferably not less than 1% by weight, more preferably not less than 3% by weight, and particularly preferably not less than 5% by weight. In light of the feel at impact of the golf ball 2, the content is preferably not greater than 50% by weight, more preferably not greater than 47% by weight, and particularly preferably not greater than 45% by weight.

In the present embodiment, the styrene block-containing thermoplastic elastomer includes an alloy of an olefin and at least one selected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, and SEEPS. The olefin component in the alloy is presumed to contribute to improvement of compatibility with another base polymer. The alloy can contribute to the resilience performance of the golf ball 2. An olefin having 2 to 10 carbon atoms is preferred. Examples of suitable olefins include ethylene, propylene, butene, and pentene. Ethylene and propylene are particularly preferred.

Specific examples of the polymer alloy include trade names “TEFABLOC T3221C”, “TEFABLOC T3339C”, “TEFABLOC SJ4400N”, “TEFABLOC SJ5400N”, “TEFABLOC SJ6400N”, “TEFABLOC SJ7400N”, “TEFABLOC SJ8400N”, “TEFABLOC SJ9400N”, and “TEFABLOC SR04”, manufactured by Mitsubishi Chemical Corporation. Other specific examples of the styrene block-containing thermoplastic elastomer include trade names “Epofriend A1010” manufactured by Daicel Corporation and “SEPTON HG-252” manufactured by Kuraray Co., Ltd.

The resin composition of the inner cover 6 may include a coloring agent, a filler, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and any other additive in appropriate amounts. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

The inner cover 6 preferably has a thickness Ti of not less than 0.5 mm and not greater than 2.0 mm. The golf ball 2 in which the thickness Ti is not less than 0.5 mm is excellent in durability. From this viewpoint, the thickness Ti is more preferably not less than 0.70 mm and particularly preferably not less than 0.9 mm. The golf ball 2 in which the thickness Ti is not greater than 2.00 mm is excellent in feel at impact. From this viewpoint, the thickness Ti is more preferably not greater than 1.8 mm and particularly preferably not greater than 1.6 mm. The thickness Ti is measured at a point immediately below the land 12.

The inner cover 6 preferably has a hardness Hi of not less than 60 and not greater than 100. The golf ball 2 in which the hardness Hi is not less than 60 is excellent in resilience performance. From this viewpoint, the hardness Hi is more preferably not less than 70 and particularly preferably not less than 75. The golf ball 2 in which the hardness Hi is not greater than 100 is excellent in feel at impact. From this viewpoint, the hardness Hi is more preferably not greater than 98 and particularly preferably not greater than 96.

The hardness Hi of the inner cover 6 is measured according to the standards of “ASTM-D 2240-68”. The hardness Hi is measured with a Shore C type hardness scale mounted to an automated hardness meter (“digi test II” manufactured by Heinrich Bareiss Prufgeratebau GmbH). The measurement is performed using an about 2-mm-thick sheet formed by hot press and made of the same material as the inner cover 6. The sheet is stored at 23° C. for two weeks prior to the measurement. Three such sheets are stacked at the time of measurement.

The outer cover 8 is positioned outside the inner cover 6. In the present embodiment, the outer cover 8 is in contact with the inner cover 6. The golf ball 2 may include an adhesive layer located between the inner cover 6 and the outer cover 8. The outer cover 8 can be firmly joined to the inner cover 6 via the adhesive layer. The outer cover 8 is formed from a resin composition. In the present embodiment, the outer cover 8 is formed from a thermoplastic resin composition. Examples of the base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. Ionomer resins are particularly preferred. Ionomer resins are highly elastic. The golf ball 2 that includes the outer cover 8 containing an ionomer resin is excellent in resilience performance. The golf ball 2 is excellent in flight performance in driver shots. Ionomer resins as mentioned above for the inner cover 6 can be used for the outer cover 8.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is used as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably not less than 50% by weight, more preferably not less than 60% by weight, and particularly preferably not less than 65% by weight.

A preferred resin that can be used in combination with the ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer can contribute to the feel at impact of the golf ball 2. In light of feel at impact, the proportion of the styrene block-containing thermoplastic elastomer to the entire base polymer is preferably not less than 10% by weight, more preferably not less than 20% by weight, and particularly preferably not less than 25% by weight. Styrene block-containing thermoplastic elastomers as mentioned above for the inner cover 6 can be used for the outer cover 8.

Another resin suitable for the outer cover 8 is a polyurethane elastomer. The resin composition of the outer cover 8 may include a thermoplastic or thermosetting polyurethane. The thermoplastic polyurethane is preferred in light of the efficiency of production of the golf ball 2. The thermoplastic polyurethane includes a polyurethane component as a hard segment and a polyester or polyether component as a soft segment. The thermoplastic polyurethane is flexible. The outer cover 8 in which this polyurethane is used is excellent in controllability and scuff resistance.

The thermoplastic polyurethane has a urethane bond in the molecule. The urethane bond can be formed by a reaction between a polyol and a polyisocyanate. The polyol as a material for the urethane bond has a plurality of hydroxyl groups. Low-molecular-weight polyols and high-molecular-weight polyols can be used.

Examples of the low-molecular-weight polyols include diols, triols, tetraols, and hexaols. Specific examples of the diols include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol, neopentyl glycol, pentanediol, hexanediol, heptanediol, octanediol, and 1,6-cyclohexanedimethylol. Aniline diols or bisphenol A diols may be used. Specific examples of the triols include glycerin, trimethylol propane, and hexanetriol. Specific examples of the tetraols include pentaerythritol and sorbitol.

Examples of the high-molecular-weight polyols include: polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polytetramethylene ether glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-s-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. Two or more polyols may be used in combination. In light of the feel at impact of the golf ball 2, the number average molecular weight of the high-molecular-weight polyol used is preferably not less than 400 and more preferably not less than 1000. The number average molecular weight is preferably not greater than 10000.

Examples of the polyisocyanate as a material for the urethane bond include aromatic diisocyanates, alicyclic diisocyanates, and aliphatic diisocyanates. Two or more diisocyanates may be used in combination.

Examples of the aromatic diisocyanates include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), and paraphenylene diisocyanate (PPDI). An example of the aliphatic diisocyanates is hexamethylene diisocyanate (HDI). Examples of the alicyclic diisocyanates include 4,4′-dicyclohexylmethane diisocyanate (Hi₁₂MDI), 1,3-bis(isocyanatomethyl)cyclohexane isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate (CHDI). 4,4′-Dicyclohexylmethane diisocyanate is preferred.

Specific examples of the thermoplastic polyurethane include trade names “Elastollan NY80A”, “Elastollan NY82A”, “Elastollan NY83A”, “Elastollan NY84A”, “Elastollan NY85A”, “Elastollan NY88A”, “Elastollan NY90A”, “Elastollan NY95A”, “Elastollan NY97A”, “Elastollan NY585”, and “Elastollan KP016N”, manufactured by BASF Japan Ltd.; and trade names “RESAMINE P4585LS” and “RESAMINE PS62490”, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

The resin composition of the outer cover 8 may include a coloring agent, a filler, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and any other additive in appropriate amounts. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

The outer cover 8 preferably has a thickness To of not less than 0.5 mm and not greater than 2.0 mm. The golf ball 2 in which the thickness To is not less than 0.50 mm is excellent in durability. From this viewpoint, the thickness To is more preferably not less than 0.7 mm and particularly preferably not less than 0.9 mm. The golf ball 2 in which the thickness To is not greater than 2.0 mm is excellent in feel at impact. From this viewpoint, the thickness To is more preferably not greater than 1.8 mm and particularly preferably not greater than 1.6 mm. The thickness To is measured at a point immediately below the land 12.

The outer cover 8 preferably has a hardness Ho of not less than 40 and not greater than 96. The golf ball 2 in which the hardness Ho is not less than 40 is excellent in resilience performance. From this viewpoint, the hardness Ho is more preferably not less than 50 and particularly preferably not less than 55. The golf ball 2 in which the hardness Ho is not greater than 96 is excellent in feel at impact. From this viewpoint, the hardness Ho is more preferably not greater than 90 and particularly preferably not greater than 85.

The hardness Ho of the outer cover 8 is measured in the same manner as the hardness Hi of the inner cover 6.

The total thickness (Ti+To), which is the sum of the thickness Ti of the inner cover 6 and the thickness To of the outer cover 8, is preferably not less than 1.6 mm and not greater than 3.4 mm. The golf ball 2 in which the total thickness (Ti+To) is not less than 1.6 mm is excellent in durability. From this viewpoint, the total thickness (Ti+To) is more preferably not less than 1.9 mm and particularly preferably not less than 2.0 mm. The golf ball 2 in which the total thickness (Ti+To) is not greater than 3.4 mm is excellent in feel at impact. From this viewpoint, the total thickness (Ti+To) is more preferably not greater than 3.1 mm and particularly preferably not greater than 3.0 mm.

The golf ball 2 preferably has an amount of compressive deformation Db of not less than 2.75 mm. The golf ball 2 having an amount of compressive deformation Db of not less than 2.75 mm is excellent in feel at impact. The golf ball 2 having an amount of compressive deformation Db of not less than 2.75 mm is excellent in feel at impact, especially in driver shots. From this viewpoint, the amount of compressive deformation Db is more preferably not less than 2.80 mm and particularly preferably not less than 2.85 mm. In light of resilience performance, the amount of compressive deformation Db is preferably not greater than 4.0 mm.

The amount of compressive deformation Db is measured using a YAMADA type compression tester “SCH”. In the tester, the golf ball 2 is placed on a hard plate made of metal. A cylinder made of metal gradually descends toward the golf ball 2. The golf ball 2 is squeezed between the bottom face of the cylinder and the hard plate and thus deformed. The distance is measured through which the cylinder moves from when an initial load of 98 N is applied to the golf ball 2 to when a final load of 1274 N is applied to the golf ball 2. The moving speed of the cylinder is 0.83 mm/s until the initial load is applied. The moving speed of the cylinder is 1.67 mm/s after the initial load is applied and until the final load is applied.

The amount of deformation of the golf ball 2 in approach shots is small. Mainly the outer cover 8 contributes to the feel at impact in approach shots. In the present embodiment, the hardness Ho of the outer cover 8 is smaller than the hardness Hi of the inner cover 6. The golf ball 2 having a small hardness Ho is excellent in feel at impact in approach shots.

In light of both the flight performance in driver shots and the feel at impact in approach shots, the difference (Hi−Ho) between the hardness Hi and the hardness Ho is preferably not less than 5, more preferably not less than 10, and particularly preferably not less than 13. The difference (Hi−Ho) is preferably not greater than the 40.

The product Dc*Rc of the amount of compressive deformation Dc and radius Rc of the core 4 is an index correlated with the degree of contribution of the core 4 to the stiffness of the golf ball 2. The product Hi*Ti of the hardness Hi and thickness Ti of the inner cover 6 is an index correlated with the degree of contribution of the inner cover 6 to the stiffness of the golf ball 2. The product Ho*To of the hardness Ho and thickness To of the outer cover 8 is an index correlated with the degree of contribution of the outer cover 8 to the stiffness of the golf ball 2.

The difference (Hi*Ti−Dc*Rc) between the product Hi*Ti in the inner cover 6 and the product Dc*Rc in the core 4 is not less than 5 and not greater than 40. In other words, the golf ball 2 satisfies the following mathematical formula (1).

5≤(Hi*Ti−Dc*Rc)≤40  (1);

The difference (Hi*Ti−Ho*To) between the product Hi*Ti in the inner cover 6 and the product Ho*To in the outer cover 8 is not less than 5 and not greater than 40. In other words, the golf ball 2 satisfies the following mathematical formula (2).

5≤(Hi*Ti−Ho*To)≤40  (2); and

The difference (Ho*To−Dc*Rc) between the product Ho*To in the outer cover 8 and the product Dc*Rc in the core 4 is not less than −20 and not greater than 20. In other words, the golf ball 2 satisfies the following mathematical formula (3).

−20≤(Ho*To−Dc*Rc)≤20  (3).

In the golf ball 2 satisfying the mathematical formulas (1), (2), and (3), the core 4 has low stiffness, the inner cover 6 has high stiffness, and the outer cover 8 has low stiffness. In the golf ball 2, the high-stiffness portion (inner cover 6) is interposed between the two low-stiffness portions (core 4 and outer cover 8).

In driver shots of the golf ball 2, the amount of elastic deformation of the core 4 is large, the amount of elastic deformation of the inner cover 6 is small, and the amount of elastic deformation of the outer cover 8 is large. In driver shots, the core 4 and the inner cover 6 contribute to a large launch angle and a low spin rate. Thus, the golf ball 2 can exhibit a long flight distance in driver shots. In driver shots, the outer cover 8 offers soft feel at impact. The golf ball 2 is excellent in flight performance and feel at impact in driver shots.

In approach shots, the kinetic energy transferred to the golf ball 2 from the golf club is low. In approach shots, mainly the outer cover 8 is elastically deformed. Since the outer cover 8 has low stiffness, the amount of elastic deformation of the outer cover 8 is large in approach shots. The outer cover 8 allows for a high spin rate in approach shots. The outer cover 8 contributes to controllability in approach shots. Furthermore, in approach shots, the outer cover 8 offers soft feel at impact. The golf ball 2 is excellent in controllability and feel at impact in approach shots.

The golf ball 2 satisfying the mathematical formulas (1), (2), and (3) is excellent in all of the following properties:

(a) flight performance in driver shots;

(b) feel at impact in driver shots;

(c) controllability in approach shots; and

(d) feel at impact in approach shots.

The golf ball 2 is excellent in overall performance.

In light of overall performance, the difference (Hi*Ti−Dc*Rc) is more preferably not less than 8 and particularly preferably not less than 10. In light of overall performance, the difference (Hi*Ti−Dc*Rc) is more preferably not greater than 35 and particularly preferably not greater than 32.

In light of overall performance, the difference (Hi*Ti−Ho*To) is more preferably not less than 8 and particularly preferably not less than 10. In light of overall performance, the difference (Hi*Ti−Ho*To) is more preferably not greater than 35 and particularly preferably not greater than 32.

In light of overall performance, the difference (Ho*To−Dc*Rc) is more preferably not less than −15 and particularly preferably not less than −12. In light of overall performance, the difference (Ho*To−Dc*Rc) is more preferably not greater than 15 and particularly preferably not greater than 12.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of high cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), an appropriate amount of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 2.0 parts by weight of benzoic acid, 0.5 parts by weight of diphenyl disulfide, and 0.9 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold made up of upper and lower mold halves each having a hemispherical cavity, and was heated to obtain a core with a diameter of 38.5 mm. The amount of zinc diacrylate was adjusted to give a predetermined amount of compressive deformation Dc. The amount of barium sulfate was adjusted to give a predetermined weight of the core. The crosslinking temperature was 160° C. The crosslinking time period was 20 minutes.

A resin composition C7 was obtained by kneading 50 parts by weight of an ionomer resin (“Himilan AM7329” mentioned above), 50 parts by weight of another ionomer resin (“Himilan 1605” mentioned above), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of a light stabilizer (trade name “JF-90”, manufactured by Johoku Chemical Co., Ltd.) with a twin-screw kneading extruder. The core was placed into a mold made up of upper and lower mold halves each having a hemispherical cavity. The core was covered with the resin composition C7 by injection molding to form an inner cover. The thickness of the inner cover was 1.05 mm.

A resin composition C4 was obtained by kneading 38.5 parts by weight of an ionomer resin (“Himilan AM7337” mentioned above), 38.5 parts by weight of another ionomer resin (“Himilan AM7329” mentioned above), 23 parts by weight of a styrene block-containing thermoplastic elastomer (“TEFABLOC T3221C” mentioned above), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of a light stabilizer (“JF-90” mentioned above) with a twin-screw kneading extruder. The sphere consisting of the core and inner cover was placed into a mold made up of upper and lower mold halves each having a hemispherical cavity. The sphere was covered with the resin composition C4 by injection molding to form an outer cover. The thickness of the outer cover was 1.05 mm.

A clear paint containing a two-component curable polyurethane as a base material was applied to the outer cover to obtain a golf ball of Example 1, which had a diameter of about 42.7 mm and a weight of about 45.6 g.

Examples 2 to 8 and Comparative Examples 1 to 7

Golf balls of Examples 2 to 8 and Comparative Examples 1 to 7 were obtained in the same manner as the golf ball of Example 1, except that the specifications of the core, inner cover, and outer cover were as shown in Tables 2 to 4 below. The details of the compositions of the inner and outer covers are shown in Table 1 below.

[Flight Performance W#1]

A driver (W#1) was attached to a swing machine manufactured by Golf Laboratories, Inc. The head of the driver has a body having an opening and a face closing the opening. The face has a main portion and an edge portion. The edge portion is welded to the main portion. The size in front-back direction of the edge portion is not greater than 6 mm. The head further has a weld bead located at the boundary between the main and edge portions. The thickness of the head is greater at the face-side end of the weld bead than at the head-side end of the weld bead. Each golf ball was hit with this driver at a head speed of 40 m/sec, and the flight distance was measured. The flight distance is a distance from the launch point to the stop point. The weather was almost windless during the test. The average value of data obtained by 12 measurements was calculated. The average value was rated based on the following criteria.

-   -   A: 200.0 m or more     -   B: 198.5 to less than 200.0 m     -   C: 197.0 to less than 198.5 m     -   D: Less than 197.0 m

The results are shown in Tables 2 to 4 below.

[Feel at Impact W#1]

Twenty golf players were caused to hit each golf ball with the driver described above. These players were asked about the feel at impact. The number of the players who answered that the feel at impact was soft was ranked based on the following criteria.

-   -   A: 16 or more players     -   B: 10 to 15 players     -   C: 3 to 9 players     -   D: 2 or less players

The results are shown in Tables 2 to 4 below.

[Flight Performance Wedge]

A wedge (trade name “558 RTX 2.0 Tour Satin Wedge” manufactured by Cleveland Golf, shaft hardness: S, loft angle: 52°) was attached to a swing machine manufactured by Golf Laboratories, Inc. Each golf ball was hit with this wedge at a head speed of 16 m/sec, and the spin rate was measured. The average value of data obtained by 12 measurements was calculated. The average value was rated based on the following criteria.

-   -   A: 4300 rpm or more     -   B: 4150 to less than 4300 rpm     -   C: 4000 to less than 4150 rpm     -   D: Less than 4000 rpm

The results are shown in Tables 2 to 4 below.

[Feel at Impact Wedge]

Twenty golf players were caused to hit each golf ball with the wedge described above. These players were asked about the feel at impact. The number of the players who answered that the feel at impact was soft was ranked based on the following criteria.

-   -   A: 16 or more players     -   B: 10 to 15 players     -   C: 3 to 9 players     -   D: 2 or less players

The results are shown in Tables 2 to 4 below.

[Overall Evaluation]

The overall performance was rated based on the evaluation results of the four items described above.

Overall performance A: The number of the ratings “A” is four.

Overall performance B: The requirement for overall performance A is not met. The rating was “A” or “B” in each item.

Overall performance C: The requirement for overall performance B is not met. The number of the ratings “D” is zero.

Overall performance D: The requirement for overall performance C is not met.

TABLE 1 Compositions of Covers (parts by weight) C1 C2 C3 C4 C5 C6 C7 Himilan AM7337 30 38.5 43 Himilan AM7329 30 38.5 43 55 50 Himilan 1605 50 Himilan 1555 45 TEFABLOC T3221C 40 23 14 Elastollan NY84A 100 Elastollan NY88A 100 Titanium dioxide 4 4 4 4 4 4 4 JF-90 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Hardness (Shore C) 51 58 63 76 83 92 96

TABLE 2 Evaluation Results Ex. Ex. Ex. Com. Com. 1 2 3 1 2 Core Rc (mm) 19.25 19.25 19.25 19.25 19.25 Dc (mm) 4.15 3.75 3.75 3.00 4.15 Inner cover C7 C6 C7 C7 C7 Ti (mm) 1.05 1.05 1.05 1.05 1.05 Hi (Shore C) 96 92 96 96 96 Outer cover C4 C4 C4 C4 C1 Base material Io Io Io Io Ur To (mm) 1.05 1.05 1.05 1.05 1.05 Ho (Shore C) 76 76 76 76 51 Ball Db (mm) 3.15 2.85 2.75 2.00 3.35 Dc*Rc 80 72 72 58 80 Hi*Ti 101 97 101 101 101 Ho*To 80 80 80 80 54 Hi*Ti − Dc*Rc 21 24 29 43 21 Hi*Ti − Ho*To 21 17 21 21 47 Ho*To − Dc*Rc 0 8 8 22 −26 W#1 Flight distance A B C C D Feel at impact A B C D A Wedge Spin A A B B A Feel at impact A A B D A Overall performance A B C D D

TABLE 3 Evaluation Results Ex. Ex. Com. Ex. Ex . 4 5 3 6 7 Core Rc (mm) 19.25 19.25 19.00 19.25 19.25 Dc (mm) 4.15 4.15 3.75 4.15 4.15 Inner cover C7 C7 C4 C5 C6 Ti (mm) 1.05 1.05 1.05 1.05 1.05 Hi (Shore C) 96 96 76 83 92 Outer cover C2 C3 C2 C4 C4 Base material Ur Io Io Io Io To (mm) 1.05 1.05 1.30 1.05 1.05 Ho (Shore C) 58 63 58 76 76 Ball Db (mm) 3.30 3.25 3.20 3.35 3.25 Dc*Rc 80 80 71 80 80 Hi*Ti 101 101 80 87 97 Ho*To 61 66 75 80 80 Hi*Ti − Dc*Rc 21 21 9 7 17 Hi*Ti − Ho*To 40 35 4 7 17 Ho*To − Dc*Rc −19 −14 4 0 0 W#1 Flight distance C B D C B Feel at impact A A A A A Wedge Spin A A A A A Feel at impact A A A A A Overall performance C B D C B

TABLE 4 Evaluation Results Com. Com. Com. Com. Ex. 4 5 6 7 8 Core Rc (mm) 19.00 19.00 19.25 19.00 19.00 Dc (mm) 4.15 4.15 4.15 4.15 4.15 Inner cover C7 C7 C7 C5 C5 Ti (mm) 1.05 1.30 1.55 1.05 1.30 Hi (Shore C) 96 96 96 83 83 Outer cover C4 C4 C4 C6 C6 Base material Io Io Io Io Io To (mm) 1.30 1.05 0.55 1.30 1.05 Ho (Shore C) 76 76 76 92 92 Ball Sb (mm) 3.10 2.95 2.90 3.05 3.05 Dc*Rc 79 79 80 79 79 Hi*Ti 101 125 149 87 108 Ho*To 99 80 42 120 97 Hi*Ti − Dc*Rc 22 46 69 8 29 Hi*Ti − Ho*To 2 45 107 −32 11 Ho*To − Dc − Rc 20 1 −38 41 18 W#1 Flight distance D A A A A Feel at impact A C D C B Wedge Spin A B C D C Feel at impact A D D D C Overall performance D D D D C

Notes for Tables 2 to 4

Io: The base material of the composition is an ionomer resin.

Ur: The base material of the composition is a urethane resin.

As shown in Tables 2 to 4, the golf balls of Examples had excellent overall performance. The evaluation results demonstrate the superiority of the golf ball disclosed in the present specification.

[Disclosed Items]

The following items are disclosures of preferred embodiments.

[Item 1]

A golf ball including:

a core;

an inner cover positioned outside the core; and

an outer cover positioned outside the inner cover, wherein

the golf ball satisfies the following mathematical formulas (1), (2), and (3):

5≤(Hi*Ti−Dc*Rc)≤40  (1);

5≤(Hi*Ti−Ho*To)≤40  (2); and

−20≤(Ho*To−Dc*Rc)≤20  (3), wherein

Dc is an amount of compressive deformation (mm) of the core,

Rc is a radius (mm) of the core,

Hi is a Shore C hardness of the inner cover,

Ti is a thickness (mm) of the inner cover,

Ho is a Shore C hardness of the outer cover, and

To is a thickness (mm) of the outer cover.

[Item 2]

The golf ball according to Item 1, wherein the hardness Ho is lower than the hardness Hi.

[Item 3]

The golf ball according to Item 2, wherein a difference (Hi−Ho) between the hardness Hi and the hardness Ho is not less than 5.

[Item 4]

The golf ball according to any one of Items 1 to 3, wherein an amount of compressive deformation Db of the golf ball is not less than 2.75 mm.

[Item 5]

The golf ball according to any one of Items 1 to 4, wherein

the inner cover is formed from a resin composition, and

a base material of the resin composition is an ionomer resin.

[Item 6]

The golf ball according to any one of Items 1 to 5, wherein

the outer cover is formed from a resin composition, and

a base material of the resin composition is an ionomer resin.

[Item 7]

The golf ball according to any one of Items 1 to 6, wherein the difference (Hi*Ti−Dc*Rc) is not less than 17 and not greater than 24.

[Item 8]

The golf ball according to any one of Items 1 to 7, wherein the difference (Hi*Ti−Ho*To) is not less than 17 and not greater than 35.

[Item 9]

The golf ball according to any one of Items 1 to 8, wherein the difference (Ho*To−Dc*Rc) is not less than −14 and not greater than 8.

The golf ball disclosed in the present specification is suitable for, for example, playing golf on golf courses and practicing at driving ranges. The foregoing description is given for illustrative purposes only, and various modifications can be made without departing from the principles of the present disclosure. 

What is claimed is:
 1. A golf ball comprising: a core; an inner cover positioned outside the core; and an outer cover positioned outside the inner cover, wherein the golf ball satisfies the following mathematical formulas (1), (2), and (3): 5≤(Hi*Ti−Dc*Rc)≤40  (1); 5≤(Hi*Ti−Ho*To)≤40  (2); and −20≤(Ho*To−Dc*Rc)≤20  (3), wherein Dc is an amount of compressive deformation (mm) of the core, Rc is a radius (mm) of the core, Hi is a Shore C hardness of the inner cover, Ti is a thickness (mm) of the inner cover, Ho is a Shore C hardness of the outer cover, and To is a thickness (mm) of the outer cover.
 2. The golf ball according to claim 1, wherein the hardness Ho is lower than the hardness Hi.
 3. The golf ball according to claim 2, wherein a difference (Hi−Ho) between the hardness Hi and the hardness Ho is not less than
 5. 4. The golf ball according to claim 1, wherein an amount of compressive deformation Db of the golf ball is not less than 2.75 mm.
 5. The golf ball according to claim 1, wherein the inner cover is formed from a resin composition, and a base material of the resin composition is an ionomer resin.
 6. The golf ball according to claim 1, wherein the outer cover is formed from a resin composition, and a base material of the resin composition is an ionomer resin.
 7. The golf ball according to claim 1, wherein the difference (Hi*Ti−Dc*Rc) is not less than 17 and not greater than
 24. 8. The golf ball according to claim 1, wherein the difference (Hi*Ti−Ho*To) is not less than 17 and not greater than
 35. 9. The golf ball according to claim 1, wherein the difference (Ho*To−Dc*Rc) is not less than −14 and not greater than
 8. 